I want to run magnolia CMS on openshift. are there any infrastructure prerequisite for installing magnolia cms? Eg. What should be the minimum memory on server, or space etc?
That really depends on the number of concurrent users, load of your website, etc. I have it running on a raspberry pi 2, so it runs ok on 1 GB RAM. I wouldn't say it's extremely fast though and would not recommend it for production use ;).
Also, the disk space really depends on your website size. You can check the size by just downloading Magnolia and starting the Tomcat, then check the folder size of the exploded war files. That gives you an idea of what Magnolia takes for a setup with a blank site (or the small demo site that comes with it).
Regarding disk space, also see: https://documentation.magnolia-cms.com/display/DOCS/Disk+space